Some info about child swap and the rider switch pass:
The only rides that offer child swap are those with height restrictions.
The rider switch pass (special child swap Fastpass) is given out on rides with the Fastpass option.
Rides like Pooh, HM, or Buzz that do not have Fastpass but no height requirement, don't offer the rider switch pass (special child swap Fastpass).
As far as I know there is no child swap offered at rides without a height requirement but that can, of course, depend on the CM working the entrance to the ride.
They do not let children that do not meet the height requirement through the line (except on Star Tours; the entire party enters the line and they do the switch at the ride itself).
What to do:
Approach the CM at the entrance to the attraction before you enter the ride line (either the Fastpass line or Standby line) and ask them about child swap and let them know you want to use the child swap feature and find out how they handle it; most rides handle it the same except Star Tours, as I said before.
The parent and the child must approach the CM together. They must see the child who doesn't meet the height requirement. The CM may measure the child if they are close to the height requirement. The CM will then give you a rider switch pass (if the child doesn't meet the height requirement) and the parent that is riding second can then leave with the child and to do something else while the first parent/party rides. They could wait outside or wait at the exit of the ride (which is usually in a store) or do a different attraction.
Meanwhile, the first parent/party can ride however they've decided; either Fastpass, standby or single rider, if available. Then the second parent returns, at their convenience, to ride via the Fastpass line with the rider switch pass and they can ride with up to 2 other guests; the rider switch pass is good for up to 3 guests total and is good all day.
Please note: the parent riding with the rider switch pass does not need a regular Fastpass; the rider switch pass is all they need to enter the Fastpass line. It takes the place of a regular Fastpass.
Please note: If your child is tall enough to ride but doesn't want to, then you can still create your own form of swapping by getting a Fastpass for each parent and then taking turns. Works just about the same as getting the rider switch pass. Or you could ask the CM if there's a way you could possibly swap at the ride itself or at the exit to the ride.
The purpose of child swap isn't a "head of the line pass" or to eliminate a double wait for a family that can't ride together but to shorten the wait for the second parent as much as the Fastpass line can and to make it more convenient by letting you come back later if you need/want to with up to 2 other guests to ride with you so you don't have to ride alone.
